The invention relates to a picture display device for displaying a picture by means of a line raster consisting of a plurality of lines which are successively scanned in the opposite direction, said device comprising a video signal processing circuit for receiving an incoming video signal comprising video information and synchronising signals and for supplying the video information of a line in one direction and the video information of the subsequent line in the opposite direction, and a line deflection circuit for generating a line deflection current through a line deflection coil which forms part of a resonant network for obtaining the said scanning.
A picture display device of this type is known from European Patent Application No. 175,409 (PHN 11 432). Advantages of the symmetrical line deflection described in this Application are the low dissipation power of the line deflection circuit and the low voltage load of the different elements of this circuit. The line deflection current has the frequency and the variation which are determined by the input signal of the amplifier supplying this current, which signal originates from a line synchronising circuit and is synchronised with the line synchronising signal present in the incoming video signal. A further essential reduction of the dissipation is obtained because the line deflection coil is incorporated in a resonant circuit whose tuning frequency is equal to half the line frequency, i.e. the frequency of the line synchronising signal, whilst the line deflection current flowing through the coil also has half the line frequency. However, in practice it has been found that the exact tuning is not achieved and that variations of the resonance frequency occur, for example, owing to tolerances, temperature effects or by ageing or replacement of components. Moreover, the incoming line frequency may vary, for example, when the incoming video signal originates from a video recorder. As a result of these effects a larger dissipation is required to maintain the correct amplitude of the line deflection current than in the case where the resonance frequency is always equal to half the incoming line frequency. This problem is known: the German publication "Neues aus der Technik", no. 4 of 15, Nov. 1985, page 4, proposes to control the tuning frequency of the resonant circuit by providing a controllable inductive element as part of the said circuit. However, such an element is expensive and it dissipates energy.